1. Field of the Invention
This invention relates generally to equipment for sound cinematography; and more particularly to a sound-damping lens adapter for reducing the amount of noise that is radiated from a standard camera and lens into a scene area.
2. Prior Art
In the motion-picture industry it is well known that the operation of a camera movement generates mechanical vibration which can be radiated acoustically as noise. Such noise is objectionable, because it interferes with the recording of sound from the scene area being photographed.
Manufacturers of some motion-picture cameras accordingly design the camera movements to minimize the amount of vibration generated, and also design the camera body or case to absorb as much of the generated vibration as feasible. Unfortunately, however, one route for escape of such vibration is particularly difficult to control.
In particular, vibration is transmitted mechanically from the camera through any conventional lens adapter to a lens. The glass elements of the lens, which are normally exposed to the scene area, radiate some of this vibration.
Lenses are very efficient, unfortunately, in acting as loudspeakers to radiate such noise forward. Except for their relatively mild optical curvature, such lenses have exposed glass surfaces that are essentially vertical, disposed transverse to the optical axis and lens axis.
The frontal surfaces cannot couple transverse vibration (vibration perpendicular to the axis) efficiently into the air as sound. In transverse vibration, the motion is nearly parallel to the surface. When vibrating transversely, in effect, these surfaces merely slide through the air.
For longitudinal vibration, however, the frontal lens surfaces are very efficient in coupling energy into the air In one half-cycle of the mechanical vibration, the entire lens surface 181 (FIG. 4) pushes the air forward, creating a high-pressure wavefront or peak. On another half-cycle the glass surface 181 retreats from the air, creating a low-pressure wavefront or trough.
These pressure variations propagate through the air, away from the lens, while the vibration cycle repeats, radiating a continuous series of pressure waves--noise. The noise problem created in this way is particularly severe for some of the most highly popular modern lenses, such as wide-aperture zoom lenses, that have large-diameter front elements.
The industry has used two different strategies for reducing this noise in the scene area: interception of mechanical vibration before it reaches the lens adapter, and interception of noise after it leaves the lens.
As to the first of these noise-reduction strategies, certain modern camera models commercially available under the trade name Arriflex.RTM. incorporate a built-in vibration filter or sound-damping assembly for attachment of lenses (or, more specifically, for attachment of lens adapters).
The built-in sound-damper assembly 80 (FIGS. 4 and 5) includes a frontal block 82 that is preferably of a relatively heavy metal, and of relatively massive proportions. The block 82 is secured to a resilient vibration absorber 83, as shown in FIGS. 4 and 5, and includes a lens-receiving fitting 123 in its forward face.
Such devices must be stabilized against excessive drooping and other uncontrolled motion of the lens relative to the focal plane of the camera. Stabilization and support are enhanced by providing a number of longitudinally oriented support pins or guide pins, which are fixed to the camera mount--that is, fixed to the relatively thin plate 84 at the camera end of the assembly--and which fit closely but smoothly into apertures formed in the rear of the block 82.
The support pins thus slip in a close fit in the apertures defined in the block 82. To put it another way, since it is the pins that are held stationary relative to the camera body 71, the block slips in a close fit on the pins.
Due to their longitudinal orientation, the pins transmit transverse forces between the camera mount and the block 82, and thus provide transverse stabilization and support. (The force of gravity is usually transverse to the lens axis, except for significant camera tilts.) This arrangement does allow transverse vibration to reach the lens 81, but without adverse results: as explained above, transverse vibration does not produce noise efficiently.
The pins do not, however, efficiently transmit forces that are longitudinal. In particular the pins do not efficiently transmit longitudinal vibration. These statements are accurate to the extent that friction between the pins and block can be ignored. Hence the pins do not pass on to the lens the vibration that most directly produces acoustic radiation from an exposed lens surface
The only other route for transmission of longitudinal vibration to the lens is through the resilient absorber. The resiliency of the absorber--a relatively hard rubber of durometer fifty--is designed to filter, absorb or "damp" a significant fraction of such vibration.
Through trial and error the resiliency is chosen so that the lens and block 82, by their own inertia, essentially float in a fixed longitudinal position--while the camera mount on the other side of the absorber oscillates. The resilient element expands and contracts to take up the difference. At the same time the pins slip in and out of the apertures in the block 82, giving needed transverse support but transmitting very little longitudinal vibration.
The Arriflex sound-damper assembly 80 is considered feasible because it is protected from the environment.
Even though this first noise-reduction strategy significantly reduces the noise radiated from the lens, important problems persist. First, the residual noise remains objectionably high. Furthermore, other camera models are not readily retrofitted with the built-in damping assembly.
From the drawings of the Arriflex built-in vibration-damper assembly it is possible to guess at one reason that it is not more effective. The noise damper is in a crowded part of the camera. The designers had to work with very difficult space constraints. The solutions are ingenious, but necessarily involve tradeoffs of sound-damping efficacy for various other operating criteria.
Making the frontal block larger and more massive would have improved performance--but not at all in proportion to the added weight or added space employed. The block is already of heavy metal and moderately bulky, and the weight of the block is already augmented by the weight of the adapter and lens.
Making the resilient absorber thicker might have improved performance. Again, however, the improvement probably would have been less than proportional to the added space consumption, and at the cost of severe problems with support and stability.
Therefore a straightforward "upsizing" to increase the damper effectiveness significantly would have required a very large increase in space consumption--almost surely an unacceptable option. Thus the first noise-reduction strategy is inherently limited, at least within the context of generally conventional and standard camera geometries.
As to the second noise-reduction strategy mentioned above, camera accessories called "blimp housings" are available. These housings enclose at least the entire lens--or the entire front end of the camera--so that a mechanically isolated sound-absorbing glass panel can be interposed in front of the lens.
Blimp housings too are effective in reducing noise radiation, but have not gained great acceptance in the industry. They are necessarily very large and cumbersome, interfering with both exchange and manipulation of lenses. Moreover the glass panel is an extra unwanted optical element, an added potential source of bothersome light reflections, scattering and attenuation.
Even the most modern and costly motion-picture cameras are subject to generation of objectionable noise vibration; and the most modern, costly and popular lenses are subject to transmission and radiation of such noise vibration from the camera into a scene area.